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Total Acid Number (TAN)

Definition - What does Total Acid Number (TAN) mean?

The total acid number (TAN) is a measurement of acidity that is determined by the amount of potassium hydroxide in milligrams that is needed to neutralize the acids in one gram of oil. It is used to estimate the amount of additive depletion, acidic contamination and oxidation of lubricant degradation.

It is an important quality measurement of crude oil and used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service.

The TAN value indicates to the crude oil refinery the potential of corrosion problems. Testing for TAN is essential to maintain and protect equipment, preventing damage in advance. TAN testing is a measure of both the weak organic acids and strong inorganic acids present within oil.

Corrosionpedia explains Total Acid Number (TAN)

Total acid number values specify the quantity of acidic compounds present in a petrochemical sample. The total acid number is an analytical test to determine the deterioration of lubricants—the more acidic a lubricant is, the more degradation occurs. As a fluid degrades, the levels of corrosive acids increase, along with the danger of component failure.

The standard way of determining the TAN value of a sample is by titration analysis, with the endpoint determined by either potentiometric or photometric titration.

The TAN value itself cannot be used to predict the corrosive nature of an oil, as the test only measures the amount of acid in a sample, not the specific quantities of different acidic compounds in the sample. Two samples might have the same TAN value, but one have high levels of corrosive acids while the other much lower levels of the same corrosive acids. An increase in viscosity and the formation of gums and resins are two other negative effects which can be attributed to an increased TAN value.

It is usually the naphthenic acids in the crude oil that causes corrosion problems. This type of corrosion is referred to as naphthenic acid corrosion (NAC). A rise in TAN is indicative of oil oxidation due to time and/or operating temperature. Trend as well as absolute values should be used to monitor TAN levels.